1. Field of the Invention
The present invention relates to a recording apparatus for recording digital signals, such as a digital video cassette recorder (VCR).
2. Description of the Prior Art
In a magnetic recording apparatus, such as a VCR, when reproducing, as the head goes off the track, the head output is lowered, and the error increases, so that an accurate picture cannot be reproduced. It is therefore important that the head traces the intended track accurately, that is, to keep the head tracking. In the digital VCR for home use, in particular, the tracks are narrow in order to record long programs, and it is necessary to keep the head tracking more accurately.
As the means for detecting the deviation of the head from the track, using pilot signals between tracks, the crosstalk of the pilot signal from the preceding track is compared with that of the succeeding track and a determination is made as to whether the head tracking has deviated to the preceding side or to the succeeding side.
The recording signals for this purpose are modulated when recording so as to have the frequency characteristics of three patterns F0, F1, F2 as shown in FIG. 22. In the F0 pattern, frequencies f.sub.1 and f.sub.2 possess portions of small frequency components, that is, notch portions. In the F1 pattern, frequency f1 possesses a portion having a larger frequency component, that is, a pilot signal, while frequency f.sub.2 possesses a notch portion. In the F2 pattern, frequency f.sub.1 possesses a notch portion, and frequency f.sub.2 has a pilot signal.
The recording signals are modulated so that the patterns may be in the order of F0, F1, F0, F2 as shown in FIG. 23, and recorded. When reproducing the F0 pattern, by the crosstalk of pilot signals from the F1 and F2 patterns of adjacent tracks, there is a peak in frequency components of f.sub.1 and f.sub.2. When the head is deviated from the center of the F0 pattern and is shifted to the F1 pattern side, the crosstalk of the pilot signal from, the F1 pattern becomes larger than the crosstalk talk from the F2 pattern, and therefore the frequency components of f.sub.1 of reproduced signal increase, while the frequency components of f.sub.2 decrease. Thus, by comparing the frequency components of f.sub.1 and f.sub.2 of the reproduced signals of the F0 pattern, the deviation of head tracking can be detected, and a correct tracking is realized.
Hitherto, the patterns of F0, F1, F2 are formed by controlling the linkage of "0" and "1" in the binary series to be recorded. This method is explained below. First, the input data is divided by every m bits (m: an even number), and a "0" bit is added to the beginning of the m bits to enter a pre-coder to be modulated by an interleaved NRZI (Non Return to Zero Invert) technique. Similarly, a "1" bit is added to the beginning of the m bits of the input data to enter the precoder to be modulated by the interleaved NRZI technique. The characteristic of the pre-coder is expressed by formula (1) below, and it is utilized for known partial response detection when decoding, and moreover when the polarity of the bit to be inserted is inverted, the inversion of polarity is propagated as shown in formula (1), and it causes an increase to the change of frequency characteristics by the change of polarity of the bit to be inserted. The bit of "0" or "1" to be inserted is hereinafter called a special bit (SB). EQU f.sub.k =g.sub.k +f.sub.k-2 (+is exclusive OR) (1)
where {g.sub.k } is a pre-coder input data series, and {f.sub.k } is a pre-coder output data series. Extracting the frequency components of the pre-coder output, the frequency components are compared between the pre-coder output when the SB is "0", and the pre-coder output when the SB is "1", and the precoder output closer to the desired frequency characteristic is used as the output of the recording apparatus, so that the output data series having the desired frequency characteristic is obtained.
FIG. 2 shows examples of an input data series by the interleaved NRZI technique at m=10, SB A, and output data series. As the polarity of SB A is inverted, the polarity of the second bit ahead of SB A is inverted according to formula (1). As the polarity of the second bit ahead of SB A is inverted, the polarity of the second bit ahead of the second bit ahead of SB A is inverted. In this way, as the polarity of SB A is inverted, the polarity of every odd-number bit counting from SB A is inverted. This Inversion continues until the odd-number bit from SB A is a new SB, that is, SB C. In other words, by inverting the polarity of a certain SB, the polarity of the odd-number bit counting from that SB is inverted m times. The bit of (m+1) counting is a new SB C, and this inversion is not propagated. The bit inverted as SB A is inverted are m+1 bits indicated by bullet mark.
By the interleaved NRZI and the propagation of change of SB polarity, changes of frequency characteristics by an SB polarity change are increased.
Conventionally, the frequency components were compared between the bit row of m+1 bits by interleaved NRZI modulation with "0" added as an SB to m bits, and the bit row of m+1 bits by interleaved NRZI modulation with "1" added as SB to m bits, and by recording m+1 closer to the frequency component of the desired frequency, the desired frequency component of the recorded signal was controlled. As the means for extracting frequency components, a Fourier transform is used.